Chelated hemes, which have an imidazole group covalently attached as a heme side chain, show five-coordination behavior and reversible oxygen binding as do certain hemoproteins. The effects of changes in solvent, proximal base structure, heme electronite effects, heme puckering, and steric strain upon the kinetics and equilibria of oxygen and carbon monoxide binding to these model compounds are under investigation. Diheme model compounds which display heme-heme interactions are also under similar investigations. The electronic nature of hemoprotein sites is being probed by preparing model compounds enriched in 13C, 15N and 57Fe and studying the solution behavior by nmr and other methods. Some of the sterically strained models display T-state hemoglobin behavior and the dynamic behavior of such models is under investigation.